A CD-R (CD-Recordable) is widely used as a recordable optical recording medium that is adapted to specifications of a compact disk (to be abbreviated as “CD” hereinafter). A recording capacity of the CD-R is about 680 MB. Along with a dramatic increase in an amount of information, demand for an information-recording medium with higher density and higher capacity has been increasing.
Use of laser beams for recording and reproducing which have short wavelengths can make a beam spot small, thereby making high-density optical recording possible. Lately, development of a short-wavelength semiconductor laser to be used in an optical disk system has been proceeding, and red semiconductor lasers for wavelengths of 680 nm, 660 nm, 650 nm and 635 nm are actually currently used (for example, refer to page 65 of Nikkei Electronics No. 592 published on Oct. 11, 1993). A DVD in which over two hours of motion pictures are digital-recorded by use of these semiconductor lasers has actually been used. Since the DVD is a playback-only medium, development of a recordable optical recording medium (DVD-R) corresponding to the capacity of the DVD is also under way.
In addition, development of a blue semiconductor laser for wavelengths ranging from 400 to 500 nm which makes ultra high density recording possible has been rapidly proceeding (for example, refer to page 117 of Nikkei Electronics No. 708 published on Jan. 26, 1998), and development of a recordable optical recording medium corresponding to the blue semiconductor laser is also under way.
In forming a pit by irradiating a recording layer of a recordable optical recording medium with a laser beam so as to cause a physical change and/or a chemical change in the recording layer, an optical constant and decomposition behavior of a compound are important factors for forming good pits. When the compound is not easily decomposed, sensitivity lowers, while when the compound is fiercely decomposed or susceptible to changes, influences between pits and in a radial direction become large, thereby making it difficult to form a reliable pit. When data is to be recorded on a conventional CD-R medium by use of a blue semiconductor laser wavelength, a refractive index of its recording layer is low and an extinction coefficient of the recording layer is also not an appropriate value, so that high recording properties cannot be attained. For this reason, as a compound used in the recording layer, a compound having appropriate optical properties and decomposition behavior against a blue semiconductor laser must be selected. As currently known examples of an organic dye compound for recording with a blue semiconductor laser, polyene-based dye compounds described in Japanese Patent Application Laid-Open Nos. 78576/1992 and 89279/1992, a styryl-based dye compound described in Japanese Patent Application Laid-Open No. 34489/1999, an indigoid-based dye compound described in Japanese Patent Application Laid-Open No. 78239/1999, a cyanoethene-based dye compound described in Japanese Patent Application Laid-Open No. 105423/1999 and a squarylium-based dye compound described in Japanese Patent Application Laid-Open No. 110815/1999 have been proposed in addition to cyanine-based dye compounds described in Japanese Patent Application Laid-Open Nos. 74690/1992 and 40161/1994 and porphyrin-based dye compounds described in Japanese Patent Application Laid-Open Nos. 304256/1995, 304257/1995, 127174/1996, 101953/1999 and 144312/1999.
Further, a variety of optical recording media having an improved layer structure such as an optical recording medium described in Japanese Patent Application Laid-Open No. 53758/1999 which comprises a recording layer comprised essentially of a porphyrin dye or a cyanine dye as an organic dye for forming the recording layer and a metal reflecting layer composed essentially of silver and an optical recording medium described in Japanese Patent Application Laid-Open No. 203729/1999 which makes recording over two wavelength ranges possible by having a blue sensitive dye layer containing a cyanine dye which is sensitive to a blue laser as well as a red sensitive dye layer or infrared sensitive dye layer have also been proposed.
Lately, a blue-violet semiconductor laser for wavelengths ranging from 400 to 410 nm has been developed, thereby making possible ultra high density optical recording of 15 to 30 GB capacity. Thus, development of a recordable optical recording medium which is the most suitable for the wavelength laser has been increasingly active (for example, refer to page 33 of Nikkei Electronics No. 736 published on Feb. 8, 1999, page 28 of Nikkei Electronics No. 741 published on Apr. 19, 1999, page 19 of Nikkei Electronics No. 748 published on Jul. 26, 1999, and page 117 of Nikkei Electronics No. 751 published on Sep. 6, 1999). However, it is a current situation that the aforementioned optical recording media for a blue semiconductor laser are not adapted to a laser beam having a wavelength of 400 to 410 nm sufficiently. That is, the present inventors have found that the aforementioned media using organic dyes have such a problem that signals recorded on the media may not be always able to be read satisfactorily since a carrier to noise ratio (C/N) is not always a good value. It has been an urgent necessity to solve the problem and develop an optical recording medium which makes possible high density recording/reproducing using a laser beam having a wavelength of 400 to 410 nm.